1. Field of the Invention
The present invention relates to a battery assembly system for reducing the variations of the state of charge (SOC) of a plurality of storage batteries connected in series and an electric-motor vehicle system using this battery assembly system.
2. Description of the Prior Art
In order to make an effective use of the capacity of a plurality of storage batteries connected in series, a technology for reducing the variations of SOC of the plurality of storage batteries based on a terminal voltage of each of the storage batteries is known. In a conventional technology, a discharging circuit is provided to an individual storage battery one by one, and the storage battery having a high SOC is discharged by the discharging circuit and the SOC thereof is reduced and adjusted to the SOC of the storage battery having a low SOC. Thus, the variations of the SOC are reduced.
For example, JP 61 (1986)-206179A discloses a technology of providing an individual battery with a Zener diode for bypassing a charging current when a terminal voltage of a storage battery reaches a charging termination voltage. JP 08 (1996)-213055A discloses a technology of supplying a current into a by-pass circuit when a terminal voltage of an individual storage battery exceeds the upper-limit voltage. JP 10 (1998)-322925A discloses a technology including detecting the variations of the SOC of a plurality of storage batteries by statistically processing an open circuit voltage of an individual storage battery and reducing the SOC of the storage battery with high SOC by discharging the storage battery with high SOC by the use of a discharging circuit provided to each individual storage battery so as to be adjusted to the SOC of the storage battery with low SOC. Furthermore, conventional technologies judge whether or not an abnormality is present based on the variations of the terminal voltages of the plural of storage batteries.
FIG. 8 is a block diagram showing a configuration of a conventional electric-motor vehicle system 80. The electric-motor vehicle system 80 is a general hybrid electric vehicle. The electric-motor vehicle system 80 is provided with an engine 81. The engine 81 generates power by using gasoline as fuel. The power generated by the engine 81 is transmitted to a motor 92 via a transmission 14 and further transmitted to wheels 15 via a differential gear 16.
To the motor 92, a battery assembly system 90 is connected via an inverter 93. The battery assembly system 90 supplies a discharged electric power to the motor 92 via the inverter 93 in order to assist the power transmitted from the engine 81 to the motor 92 via the transmission 14 at the time of acceleration. The battery assembly system 90 receives the electric power which the motor 92 generates while generating a braking power via the inverter 93 and is charged with the received power at the time of deceleration.
The electric-motor vehicle system 80 is provided with a control ECU 85. The control ECU 85 brings the engine 81 into operation with an optimum fuel efficiency based on a vehicle information such as information about a degree of opening of an accelerator, and the like, information obtained from the engine 81, and information obtained from the battery assembly system 90, and at the same time controls the engine 81 and the inverter 93 in order to prevent the overcharge, overdischarge and overload of the storage battery provided in the battery assembly system 90.
The battery assembly system 90 judges whether the electric-motor vehicle system 80 is running or parking or stopping based on a signal from a control ECU 85. When the electric-motor vehicle system 80 is running, the control ECU 85 controls the engine 81, inverter 93 and motor 92 so that the SOC of storage batteries provided in the battery assembly system 90 is in the range from about 20% to about 80% based on the information from the battery assembly system 90. This is carried out in order to protect the storage batteries provided in the battery assembly system 90 from being overcharged and overdischarged.
FIG. 9 is a block diagram showing a configuration of the battery assembly system 90 provided to a conventional electric-motor vehicle system 80. The battery assembly system 90 is provided with a battery assembly. The battery assembly includes n storage batteries C1, C2, . . . Cn−1 and Cn connected in series (n is an integer of 2 or more). Each of the storage batteries C1, C2, . . . Cn−1 and Cn is formed of lithium (Li) ion secondary battery and its standard capacity is 10 Ah.
The battery assembly system 90 is provided with a resistance discharger 91. The resistance discharger 91 has 7 ohms (Ω) discharging resistance R1, R2, . . . Rn−1, and Rn provided to each of the individual storage batteries C1, C2, . . . Cn−1 and Cn.
The battery assembly system 90 is provided with a voltage detector 82. The voltage detector 82 detects terminal voltages of the storage batteries C1, C2, . . . Cn−1 and Cn, respectively.
The battery assembly system 90 is provided with a multiplexer 95. The multiplexer 95 connects any one terminal of n storage batteries C1, C2, . . . Cn−1 and Cn to the voltage detector 82 in accordance with a switching signal from a battery ECU 86.
The battery assembly system 90 is provided with a current detector 4. The current detector 4 measures voltages at both ends of the resistor SH1 connected in series to n storage batteries in order to detect a current flowing in the storage batteries.
The battery assembly system 90 is provided with a temperature detector 9. The temperature detector 9 detects a temperature of the storage battery by the use of a temperature sensor.
In the thus configured battery assembly system 90, the battery ECU 86 judges whether or not the electric-motor vehicle system 80 is running based on the information from the control ECU 85. When the battery ECU 86 judges that the electric-motor vehicle system 80 is not running, the battery ECU 86 makes the battery assembly system 90 in a dormant state. This is carried out in order to minimize electric power consumption of the battery assembly system 90 by not supplying the battery assembly system 90 with the electric power from the outside when the electric-motor vehicle system 80 is not running.
When the battery ECU 86 judges that the electric-motor vehicle system 80 is running, the ECU 86 switches the multiplexer 95 at high speed of several miliseconds/channel (CH) or less so that the voltage detector 82 detects the terminal voltages of the storage batteries C1, C2, . . . Cn−1 and Cn, respectively. Since the terminal voltage of the storage battery is susceptible to the load fluctuation, the detection of the terminal voltages of the storage batteries C1, C2, . . . Cn−1 and Cn should be finished while the load fluctuation is small, or while the electric-motor vehicle system 80 is stopping. Then, the temperature detector 9 detects temperatures of the storage battery at an appropriate period.
Next, the battery ECU 86 outputs charging request, discharging request, charging permission, or discharging permission to the control ECU 85 based on the terminal voltage of each of the storage batteries C1, C2, . . . Cn−1 and Cn detected by the voltage detector 82. The discharging permission is limited by a storage battery having the lowest terminal voltage, that is, a storage battery having the lowest SOC. The charging permission is limited by a storage battery having the highest terminal voltage, that is, a storage battery having the highest SOC. The battery ECU 86 is operated by a driving electric power supplied from each of the storage batteries C1, C2, . . . Cn−1 and Cn.
FIG. 10 is a graph to illustrate the relationship between the variations of the SOC and the range of usable SOC of each storage battery provided to the conventional battery assembly system 90. The abscissa shows the SOC of the storage battery; and the ordinate shows a terminal voltage of the storage battery. When the variations 17 of the individual storage batteries constituting the battery assembly are expanded to the variations 18, the interval 19 between the SOC in the state of the charging permission and the SOC in the state of the discharging permission is reduced to the interval 20. Therefore, the capacity of the battery assembly that can be used as the battery assembly system is reduced. That is, the region capable of charging/discharging storage battery is reduced.
The SOC shows 50%±20% normal distribution. In order to avoid the above-mentioned situation, the battery ECU 86 controls switches provided between resisters R1, R2, . . . and Rn and storage batteries C1, C2, . . . and Cn, respectively, and equalizes the terminal voltages of the storage batteries. Thus, the SOC of the storage battery can be equalized. As a result, the capacity that can be used as the battery assembly is restored.
Specifically, the battery ECU 86 appropriately turns the switches connected respectively to the resisters R1, R2, . . . and Rn ON based on the terminal voltages of each of the storage batteries C1, C2, . . . and Cn detected by the voltage detector 82. However, the switch connected to the storage battery having the minimum terminal voltage is not turned on.
Then, the detection of the terminal voltage of each storage battery continues. In order to stop the discharge of the storage battery whose terminal voltage is equal to the minimum terminal voltage, the switch connected to the storage battery is turned OFF. When the discharge is stopped, since the polarization by discharge is dissolved, the terminal voltage is increased. Then, the switch connected to the storage battery is turned ON again.
FIG. 11 is a graph showing a distribution of the SOC of storage batteries before being equalized by the resistance discharger 91 provided for the conventional battery assembly system 90. FIG. 12 is a graph showing the distribution of the SOC after being equalized. By repeating the above-mentioned operations, the terminal voltage of the storage batteries is converged from the voltage having the variations shown in FIG. 11 to the minimum terminal voltage shown in FIG. 12.
In this conventional technology, the discharge current is set to be about 1/50C. Even if the vehicle is operated for only about an hour a week, the discharge current of 1/50C is able to correct a shift in battery capacity that normally is seen in the course of one year. The equalization ability of this level is necessary from the conventional actual result in the market.
However, since the above-mentioned switching element provided to the resister discharging circuit for equalization requires the leakage current of several tens of micro amperes (μA) or less and the control current of about the level of several hundred milli amperes (mA), the switching element is expensive. Furthermore, it is also necessary to wire the switching element with the discharge resistor and the storage battery.
Furthermore, in the above-mentioned conventional configuration, since it is necessary to provide a discharging resistor for each storage battery, the configuration of the circuit of the assembly battery system becomes complicated, and the size is increased. Therefore, there is a problem that the cost of the battery assembly system is increased.
Furthermore, since the SOC is equalized by discharging the storage battery having high SOC so as to become the same SOC as that of the storage battery having the minimum SOC, it is necessary to discharge a large amount of electricity with respect to the battery assembly as a whole. Therefore, there is a problem that an energy loss is large.
Furthermore, in the method for bypassing the discharging current during charging, the period in which a discharger can be driven is limited, and it is necessary to provide a large size discharger capable of flowing the discharge current of 1/50C or more. The capacity of the discharging storage battery is about 10 Ah and in this case, the discharging current becomes 0.5 amperes (A) or more. Therefore, there is a problem that the discharging current of the discharger becomes larger, and it is necessary to provide a large-size current element in order to turn a large current ON/OFF.
Furthermore, in the method of discharging the storage battery having high terminal voltage by connecting a resister, and opening the connection when the voltage is the same, it is necessary to repeat turning the discharge ON/OFF many times so that the SOC becomes the same. Therefore, there is a problem that it takes a long time to equalize the SOC.
Furthermore, in the conventional configuration, if the electric motor vehicle system parks for a long time, a part of the storage batteries self-discharge. As a result, there is a problem that if the other batteries are discharged so as to be adjusted to the storage battery that is largely self-discharged, the other batteries are subjected to deep discharge, making it difficult to equalize the SOC.
Furthermore, a deteriorated storage battery in the short-circuit mode may be one of the causes for expanding the variations of the SOC. When the deterioration of the storage battery advances, the variation of the SOC is expanded at higher speed than at the speed of the ability of the SOC equalization circuit. Although the method for detecting the deteriorated battery due to the variations of the terminal voltage of the storage battery has been proposed, the variations of the terminal voltage are reduced by such a current by-pass circuit and the deteriorated battery cannot be detected until the deterioration is quite advanced. Therefore, there is a problem that it is difficult to secure the battery assembly system.
It is an object of the present invention to provide a battery assembly system capable of equalizing the SOC of the battery assembly by a simple configuration and an electric motor vehicle system using the same.